1. Field of the Invention
The instant invention relates to water-based adhesive compositions, and more particularly, to film-forming vinylidene chloride copolymer-containing adhesives for use with polymer films, especially ionomer films containing carboxylate functionalities neutralized by metal ions. Such adhesives are especially useful as laminating adhesives for providing laminated constructions of ionomer carboxylate-containing hydrophobic films, especially carboxylated polyethylene films, wherein said constructions are characterized by low grease transmission and high hot tack provided by the ionomer films as well as low moisture vapor transmission and low oxygen transmission provided by the adhesive coating.
2. Description of the Art
It is well known that hydrophobic films may be laminated with solvent based adhesive systems such as the one and two component urethane adhesives and the solvent based vinylidene chloride copolymer adhesives known in the art. It is also well known that there are certain difficulties attendant with the use of such solvent based systems. For example, at least a major portion of the solvent must be removed from the adhesive after application to form a functional adhesive layer, which in turn necessitates additional equipment to ensure that the solvent does not enter the environment or affect persons applying the adhesive.
Furthermore, although it is economically desirable to recover the solvent, even the best solvent recovery equipment operates with some loss of solvent.
Water based adhesives (or surfactant-stabilized dispersions of the adhesive in water) are more desirable since the removal of water from the applied adhesive does not encounter the difficulties associated with the known solvent based systems. Nor is it economically necessary to recover the water. Water based adhesives, however, do not generally provide sufficient adhesion of hydrophobic films such as polyolefin films, and especially carboxylated polyolefin films such as Surlyn.RTM. film, either to other hydrophobic films or to non-hydrophobic films such as polyester films, cellulose films, or nylon films. When water based adhesives are used to laminate films, they must be formulated with minimum amounts of free surfactant to approach the adhesive properties of the solvent based systems. However, it has been generally found that as the free surfactant concentration of the water based adhesives is decreased, the stability also decreases. Decreased stability is a problem in commercial operation since the adhesives must be applied at high shear rates to maximize production. Such high shear rates are known in the art to destabilize water based adhesives.
Vinylidene chloride copolymers have been used to formulate water based adhesives for laminating various substrates to hydrophobic films. In general these adhesives are utilized to take advantage of the barrier properties, i.e., low moisture vapor and oxygen transmission properties, provided by the vinylidene chloride portion of the copolymer. Therefore, vinylidene chloride copolymers having a high (i.e. up to 95 percent by weight) vinylidene chloride content have been generally used in such water based adhesives. For example, a series of patents to Dodwell and others disclose vinylidene chloride copolymers having at least 60 mole percent vinylidene chloride, and preferably at least about 80 percent by weight vinylidene chloride, as useful for adhering a hydrophilic silver halide emulsion to a hydrophobic polyester film. (See U.S. Pat. Nos. 4,287,298; 4,244,988; and 4,233,074). These patents suggest that an adhesion promoter be utilized to assist in binding the vinylidene chloride adhesive to the hydrophobic polyester film. Therefore, the high amount of vinylidene chloride taught by Dodwell does not interfere with the adhesive properties of the copolymer. See also U.S. Pat. Nos. 3,988,157 and 4,002,802, to Van Paesschen et al. and Bayless et al., respectively, for other examples of the use of vinylidene chloride copolymers in photographic film.
Some patents disclose that copolymers having a high vinylidene chloride content tend to be crystalline and therefore special techniques are required for their use as adhesives or coatings. For example, in U.S. Pat. No.3,922,451 to Anschutz et al., the vinylidene chloride copolymer is heated to a temperature of at least 110.degree. C. to adhere the copolymer to the substrate. In U.S. Pat. No. 3,908,070 to Marzolf, the vinylidene chloride copolymer is externally plasticized and applied to films by a coextrusion technique in order to obtain the necessary adhesive characteristics.
Smith et al. in U.S. Pat. No. 3,850,726 teach the use of vinylidene chloride copolymers comprising 70 to 95 percent by weight vinylidene chloride to laminate a polyolefin film to another polyolefin. Again, due to the nature of the vinylidene chloride copolymers disclosed therein, the adhesive must be heat sealed at a temperature of about 270.degree. F. to obtain a substantial adhesive bond between the polyolefin films.
The vinylidene chloride copolymers described in the above references generally include the lower alkyl acrylate ester comonomers, such as methyl, ethyl and butyl acrylate and methacrylate, in combination with relatively high amounts of vinylidene chloride. The lower alkyl acrylates somewhat increase the film-forming capability of the vinylidene chloride copolymer but do not contribute sufficient plasticizing action to yield copolymers flexible enough to be utilized in laminating very flexible films, such as polyolefins, without diminishing the low temperature flexibility of the polyolefin film. More specifically, the combination of high vinylidene chloride content and the lower alkyl acrylate comonomers results in copolymers having a glass transition temperature (Tg) greater than 0.degree. C. (The Tg is the temperature at which a polymer changes from the glassy state to an amorphous state.) Therefore at temperatures of about 0.degree. C. and less, the polyolefin film laminate may become stiff due to the above vinylidene chloride copolymer adhesives.
Vinylidene chloride copolymer compositions containing lower amounts of vinylidene chloride (and a lower crystallinity) are taught by Balloni et al. in U.S. Pat. No. 4,144,363 as useful adhesives for polyolefin films. These compositions are solvent based adhesives and therefore subject to the difficulties in use noted above.
Exceptional difficulties have been encountered in applying adhesives, including vinylidene chloride adhesives, to ionomer films such as Surlyn.RTM. film, a polyethylene film having attached carboxylic acid functionalities which are neutralized with metal ions, particularly zinc and sodium. Ionomer films such as Surlyn.RTM. film are highly desired for use by convertors because of their low cost, high hot tack and impermeability to grease, but even vinylidene chloride copolymers, i.e. copolymers having a high ratio of acrylate to vinylidene chloride monomer, suitable for bonding to both hydrophobic and nonhydrophobic substrates, do not adhere well to ionomer carboxylate-containing copolymers such as Surlyn.RTM. film. Moreover, adhesives containing sufficiently high acrylate to vinylidene chloride ratios are correspondingly deficient in the moisture barrier properties of the high vinylidene chloride-containing adhesives. The metal ions on the surface of the ionomer films prevent wetting by the polyvinylidene chloride adhesive and therefore impair adhesion thereto. Improved adhesion of polyvinylidene copolymers to films of ionomer carboxylate-containing copolymers having attached metal ions requires a method of altering the adverse effect upon adhesion of the metal ions contained on the surface of the ionomer films.
Therefore, it is the primary object of this invention to provide a film-forming water based adhesive composition suitable for laminating ionomer hydrophobic films containing carboxylate components neutralized by metal ions.
It is another object of the invention to provide water based film-forming vinylidene chloride copolymers suitable for use with hydrophobic polyethylene films containing carboxylate components which do not require heating above 250.degree. F. to be useful adhesives.
It is another object of the invention to provide laminated structures including one or more hydrophobic or non-hydrophobic films adhered to an ionomer carboxylate-containing film and having substantial delamination resistance at high humidity conditions.
A further object of the invention is to provide heat-sealed structures of two or more ionomer carboxylate-containing polyethylene films.
It is another object of the invention to provide a process for adhering ionomer carboxylate-containing polyethylene films to hydrophobic films with an aqueous vinylidene chloride copolymer adhesive.
Other objects, advantages and features of this invention will become apparent to those skilled in the art from the following description.